Coatings made up of nickel and phosphorus deposited by electroless deposition processes (NiP-coatings) are commonly used as for example corrosion-resistant coatings in the electronics industry. However, as nickel is detrimental to the environment and dangerous to consumers' health the focus recently has shifted towards new materials. Iron becomes more and more appreciated in domains that other materials dominated in the past like for example as base for coating materials since it is ubiquitous, relatively cheap and non-toxic.
However, the electroless deposition of iron based coatings proved difficult due to the formation of undesired side-products and a lack of stability of plating baths. It is well-known in the art that electroless iron deposition easily yields the formation of iron oxides, iron hydroxides and iron oxo hydroxides or other precipitates under conditions suitable for the electroless deposition of other metals.
Hitherto, it was therefore common to employ a sacrificial anode (e.g. made of aluminium) in plating solutions for deposition of binary iron alloy (e.g. iron boron) coatings. N. Fujita et al., Applied Surface Science 1997, volume 113/114, pages 61-65 teaches such a process for the deposition of binary iron boron alloys but reports that the alloy deposition stopped as soon as the electrical connection between the substrate and the sacrificial anode was interrupted. Sacrificial anodes are typically base metal substrates in forms such as wires or strips which can be used as external sources of electrons. These sacrificial anodes are therefore electrically connected with the substrate (while they may be immersed into the plating bath) and provide the electrons necessary to reduce iron on the surface of the substrate. Such a plating method is essentially an electrolytic plating process since the sacrificial anode acts as local battery. This requirement of electrical connection renders these electrolytic plating baths in need of a sacrificial anode incompatible with today's demands of miniaturization in the electronics industry where many small substrates have to be coated at the same time (which all would have to be electrically connected to a sacrificial anode). Also, non-conductive substrates cannot be used as they do not allow for any electrons to pass through them to their surface. Another example for this technology is Hu Wangyu, Zhang Bangwei, Physica B 1991, volume 175, pages 396-400. Also, Chinese patent CN 100562603 C relates to the electroless deposition of ternary rare earth-iron-boron alloys on copper foils using a sacrificial anode. The plating bath disclosed therein further requires the metallic substrate to be activated with a catalyst before plating occurs. W. Lingling et al. (in Metal Finishing 2001, volume 99 (6), pages 92-96) report ternary iron-tin-boron alloys to be depositable with the use of such sacrificial aluminium anodes.
Contrary to these electrolytic metal deposition methods using an external source of electrons electroless processes are known for the formation of films of many metals. Electroless plating is the controlled autocatalytic deposition of a continuous film of metal without the assistance of an external supply of electrons. The main components of electroless metal plating baths are the source of metal ions, a complexing agent, a reducing agent, and, as optional ingredients stabilising agents, grain refiners and pH adjustors (acids, bases, buffers). Complexing agents (also called chelating agents in the art) are used to chelate the metal to be deposited and prevent the metal from being precipitated from solution (i.e. as the hydroxide and the like). Chelating metal renders the metal available to the reducing agent which converts the metal ions to their metallic form. A further form of metal deposition is immersion plating. Immersion plating is another deposition of metal without the assistance of an external supply of electrons and without chemical reducing agent. The mechanism relies on the substitution of metals from an underlying substrate for metal ions present in the immersion plating solution. In the context of the present invention electroless plating is to be understood as autocatalytic deposition with the aid of a chemical reducing agent (referred to a “reducing agent” herein).
A possibility to form iron containing deposits by electroless processes is the deposition of ternary alloys of nickel, iron and phosphorous or boron. Such processes have been reported in U.S. Pat. No. 3,385,725 and U.S. Pat. No. 3,483,029. The deposits described therein consist mostly of nickel and contain 2% or less of phosphorous or boron. Although U.S. Pat. No. 3,385,725 teaches plating baths containing equally high amounts of nickel and iron, the deposits formed consist mostly of nickel. The disclosed methods are thus unsuitable to form deposits with high iron contents.
U.S. Pat. No. 3,150,994 relates to a method of electrolessly plating metal boron alloys onto metal surfaces. It also discloses a method to form iron boron alloys on said substrates specifically from a plating bath consisting of a large excess of ammonia, a soluble iron salt and an ionic borohydride. However, the disclosed plating is inevitably accompanied by a precipitation of the formed alloy in the bath itself and, thus, results in a limitation of the lifetime of the bath. It is particularly disadvantageous of the disclosed method that the precipitate itself is an active catalytic site which facilitates further deposition.
British patent application number GB 1339829 discloses a method to deposit transparent coatings made of iron boron alloys on window glass. A necessary prerequisite of this method is, however, the employment of a hydrazine derivative in the plating bath. This is incompatible with today's security demands due to the compound's toxic and carcinogenic potential. Also, an activation step of the substrate prior to plating is required.
British patent application number GB 1365172 teaches a prolonged lifetime of the plating bath according to the aforementioned British patent application by employing carbonyl compounds therein. However, the use of hydrazine as further reducing agent and the activation step are still necessary.
US 2009/0117285 discloses an electroless deposition method for iron boron alloys on previously activated cellulose fibres. However, this method requires a very narrow pH-operation window to be used. Also, the bath disclosed therein lacks stability and plating rate (see example 1).